Conventionally, urban areas had the ability to control traffic signals via a centralized control network to which all intersections in a given area were connected. As a result of the urban sprawl common to most cities, the majority of traffic light controlled intersections operate independently of any control network as was conventionally found in center city areas. Independent control of the traffic light controlled intersections has been made possible through the advent of embedded processor based traffic light controllers. Accordingly, a majority of the signal controlled intersections now operate on a stand-alone basis, receiving inputs from external sensors, such as, for example, in-ground magnetic vehicle sensors, and the like. Many of these stand-alone intersection controllers are also equipped to receive inputs from intersection preemption systems.
To alleviate the problems associated with getting an emergency vehicle, such as, for example, ambulance, fire engine, police car, and the like, from point A to point B over sure streets in a safe and expeditious manner, a reliable system of preempting intersection traffic control signals along the route of the emergency vehicle is required. To this end, traffic signal preemption systems should ideally provide activation of the traffic control signals at a sufficient distance under all weather conditions to allow sufficient time for the traffic signals to complete their normal cycle, clearing both pedestrians and vehicles from the intersection prior to providing a green light to the emergency vehicle. Early and reliable preemption of the intersection light is also important in dispersing vehicles stopped at the intersection in the path of the emergency vehicle due to a red light.
Existing traffic signal preemption systems that have been installed and are currently in use include a strobe light system that uses the visible light spectrum as the preemptive control link between the emergency vehicle and the intersection. A strobe light based system suffers from many disadvantages, including significant range degradation under adverse weather conditions, such as, for example, heavy fog, rain, snow, or the like. Additional degradation occurs because of the propensity of these systems to accumulate dust and dirt on the optical lens portions thereof, especially at the intersection receiver end of the system.
Another type of system that has been used is a sound activated system that detects approaching sirens and determines the direction of approach to initiate the proper preemption sequence. Sound-based systems suffer significant problems with extraneous background or ambient noise causing false alarms to the system, thereby exacerbating the intersection's function, especially where there is no emergency vehicle approaching.
U.S. Pat. No. 4,228,419 to Anderson is directed to an emergency vehicle traffic control system. The system of Anderson uses a microwave transmitter mounted on each emergency vehicle in the system. The emergency vehicle emits a pulse coded message that is received by a permanently fixed receiver at each intersection to be controlled. The receiver includes a plurality of directional antennas that face the various directions to be controlled, thereby automatically discriminating a direction of approach of the emergency vehicle. The receiver further includes circuitry to verify that the preemption request has been transmitted by an authorized user. The system is reset by an internal delay based on a time after which a preemption signal is no longer received, which permits the emergency vehicle to clear the intersection. The system of Anderson relies on bulky and aesthetically unpleasing directional horn antennas mounted on the traffic signal. Additionally, the system relies heavily on built-in delays and cannot control closely located orthogonal intersections.
Further examples of traffic signal preemption signals are found in U.S. Pat. Nos. 5,014,052 and 4,228,419 to Obeck; U.S. Pat. No. 5,345,232 to Robertson; U.S. Pat. No. 4,223,295 to Bonner et al.; U.S. Pat. No. 4,573,049 to Anderson; U.S. Pat. No. 5,083,125 to Brown et al.; and U.S. Pat. No. Re 28,100 to Long.
Those skilled in the art will understand and appreciate the shortcomings of the foregoing conventional systems that do not provide any ability of the traffic preemption system to "see" around corners and to provide sufficient time for preemption and clearing of the intersection to be controlled.